This application claims the priority of German patent document No. 199 08 905.1, filed Mar. 2, 1999, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a fuel cell system which includes a hydrogen generating arrangement for feeding the fuel cell anode with a hydrogen-containing product gas by means of a conversion reaction of a hydrocarbon or hydrocarbon derivative, such as gasoline, diesel oil or methanol, with water fed by way of a water feeding system.
Fuel cell systems of this type, with an integrated hydrogen generation system, have been used with increasing frequency in vehicles powered by fuel cells. The starting substance is then preferably carried along in liquid form, and the fuel cells can be operated very effectively by means of hydrogen, without, however, the need for voluminous and heavy hydrogen storage devices.
In a known system, a reforming arrangement is used to generate hydrogen. The starting substance is converted by an endothermal water vapor reforming reaction, which may be combined with a partial oxidation reaction to obtain an autothermal conversion process. In this case, a hydrogen-rich product gas is formed in a corresponding conversion reactor. The product gas, however, frequently still contains carbon monoxide in a proportion which would be damaging to the fuel cell anode. It is therefore known to connect a gas purification stage downstream of the reactor unit, or to integrate the gas purification stage into the reactor unit, for example, in the form of a hydrogen-separating membrane, a CO-oxidation unit for the selective CO-oxidation and/or a water gas shift reactor unit for conversion of carbon monoxide to carbon dioxide by way of a water gas balance reaction.
In the system disclosed in U.S. Pat. No. 4,365,006, the cathode waste gas of a fuel cell is fed to a hydrogen generating reactor, to which a hydrocarbon starting substance or a hydrocarbon derivative starting substance is also fed. The reactor is constructed in two stages (an inlet-side partial oxidation unit and an outlet-side vapor reforming unit), and the product gas that is generated is fed directly, without further gas purification, as a fuel gas into the fuel cell anode. The anode waste gas is utilized in a burner which is used for heating the reactor and an evaporator.
European Patent Document EP 0 642 184 A2 teaches the utilization of the cathode and the anode waste gases of a fuel cell in a catalytic burner for heating a reforming reactor. The latter utilizes a starting substance which is to be reformed (particularly natural gas) to generate a hydrogen-containing product gas that is fed to the fuel cell anode, without further gas purification.
German Patent Document DE 40 32 652 A1 discloses a fuel cell system of the type described above, in which the hydrogen generating arrangement comprises a vapor reforming reactor, which receives a starting substance such as natural gas, for example; a two-stage gas purification stage for CO reduction; a water separator, and a unit for washing out carbon dioxide. The reforming reactor is heated by the flue gas of a burner, which receives at least one portion of the cathode waste gas of the fuel cell as air oxygen supply and a branched off portion of the starting substance to be reformed and/or a portion of the anode waste gas of the fuel cell, as the fuel. A portion of the anode waste gas can also be fed to the first and second gas purification stages, respectively.
In U.S. Pat. No. 4,759,997, the waste gases of a fuel cell cathode and of a heating burner of a reforming unit assigned to the fuel cell are supplied to a catalytic supplementary burner and to drive a turbine by means of its waste gas flow. The turbine is mechanically coupled with a compressor which is used for compressing air fed to the fuel cell cathode and to the burner of the reforming unit.
One object of the present invention is to provide a fuel cell system with an integrated hydrogen generating system of the type descried above, which permits a particularly efficient utilization of water and exhibits good dynamics under fluctuating load conditions.
Another object of the invention is to provide such a fuel cell system with a relatively compact construction.
These and other objects and advantages are achieved by the fuel cell system according to the invention, which includes water recovery devices that condense water out of a process gas supplied by the hydrogen generating arrangement and/or out of the cathode waste gas of the fuel cell cathode. Water received in this manner is returned to the existing water feeding system which supplies water for the conversion reaction of the hydrocarbon starting substance or the hydrocarbon derivative starting substance in the hydrogen generating arrangement.
Recovery of water fed into the hydrogen generating arrangement and/or formed on the fuel cell cathode facilitates optimal water utilization, and thus a water balance. This feature is highly advantageous, particularly for mobile applications (for example, in fuel cell powered vehicles), because the water feeding system (for which there is in this case no tap water connection) requires only a relatively small water tank.
At the same time, the feeding of the one or several fuel cells by means of a hydrogen-containing product gas, which preferably consists of highly pure hydrogen, permits a high efficiency for the fuel cell operation, with good dynamics and compact construction.
In one embodiment of the invention, a water gas shift reactor unit is connected downstream from the conversion reactor unit, and the water feeding system feeds water to the conversion reactor unit and/or the water gas shift reactor unit. The former can be used particularly for carrying out a water vapor reforming reaction of the hydrocarbon or hydrocarbon derivative starting substance, while the feeding of water into the shift reactor unit displaces the water gas reaction equilibrium in favor of an increased CO conversion. Thus, an effective water gas reaction (also called shift reaction) can be used to reduce the CO concentration in the hydrogen-containing product gas. Furthermore, water feeding to the fuel cell cathode may be provided, for example, for cooling purposes.
In another embodiment of the invention, at least one hydrogen-separating membrane is provided in the hydrogen generating arrangement. The hydrogen separating membrane selectively separates the hydrogen-containing product gas from the remaining residual conversion gas, particularly the hydrogen used for feeding the fuel cell anode from carbon monoxide formed during the conversion reaction of the hydrocarbon or hydrocarbon derivative starting substance. Specifically, this gas purification measure can be combined with another gas purification measure, such as a water gas reaction and/or a CO oxidation. Water which may be contained in the unseparated residual conversion gas is condensed out by means of a corresponding condenser, and is therefore available for another use in the system. In addition, a water feeding line leading to the conversion reactor unit may be guided by way of this residual conversion gas water condenser so that in it, the water fed into the conversion reactor unit can be preheated.
In still another embodiment of the invention, the water recovery devices contain two condensers. One is used for condensing water out of a process gas supplied by the hydrogen generating arrangement, while the other is used for condensing water out of the fuel cell cathode waste gas. Furthermore, a common cooling circulation system is provided for the two condensers. In a mobile application in a fuel cell vehicle, the cooling circulation system may have, for example, an air-cooled cooler unit for cooling the circulation coolant. In addition, the cooling circulation system can be used for cooling the fuel cell cathode.
In still another embodiment, the hydrogen generating arrangement includes at least one hydrogen separating membrane for selective separation of the hydrogen-containing product gas from the residual conversion gas, and the water recovery devices contain the residual conversion gas water condenser. One or two heat exchangers are provided, in which the separated hydrogen-containing product gas is in thermal contact with the residual conversion gas downstream of the residual conversion gas water condenser and/or with the cathode waste gas downstream of a cathode waste gas water condenser. (Thermal contact may be provided for condensing water out of the cathode waste gas.) As a result, the separated product gas can be cooled to a desired extent in a simple manner, by the residual conversion gas and/or cathode waste gas already slightly cooled in the preceding condenser.
In a further embodiment of the fuel cell system having a hydrogen generating arrangement containing a hydrogen separating membrane, an expander unit is provided in the residual conversion gas flow. The expander unit causes a relaxation of the residual conversion gas and utilizes the energy thus recovered as mechanical driving energy for a pump unit for delivering and compressing the separated hydrogen-containing product gas to be fed into the fuel cell anode.
In yet another embodiment of the invention, the anode waste gas is apportioned by way of a mixer unit to the hydrogen-containing product gas fed to the fuel cell anode on the input side, and/or it is supplied to the hydrogen generating arrangement, particularly its conversion reactor unit. In this manner, the chemical and thermal energy contained in the anode waste gas is retained for the system.
Finally, still another embodiment of the invention includes a hydrogen generating arrangement equipped with a hydrogen separating membrane. The residual conversion gas which is not separated, together with the cathode waste gas, is catalytically burned in a corresponding burner unit. A burner waste gas expander unit connected downstream of the catalytic burner unit relaxes the burner waste gas and utilizes the recovered mechanical energy to drive a pump unit for feeding cathode air and/or for the feeding air to the hydrogen generating arrangement, particularly to its conversion reactor unit.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.